Coupled multi-core optical fiber

ABSTRACT

The present embodiment relates to a CMCF including a structure to achieve more efficient reduction in transmission loss by suppressing decrease in concentration of alkali metal due to diffusion of alkali metal. In the CMCF including a plurality of cores, a power coupling coefficient h between adjacent cores is set to 1×10 −3 /m or more, to maintain an optical coupling state between the adjacent cores. In addition, alkali metal contributing to reduction in transmission loss is added to each of the cores such that a stress maximum value σ_   max    between adjacent cores has a negative value.

This is a continuation application of copending prior application Ser.No. 15/451,922, filed on Mar. 7, 2017, which is incorporated byreference herein in its entirety.

BACKGROUND

Technical Field

The present invention relates to a coupled multi-core optical fiber(hereinafter simply referred to as “CMCF”) enabling mode divisionmultiplex transmission while maintaining an optical coupled conditionbetween cores adjacent to each other.

Related Background Art

Japanese Patent Application Laid-Open No. 2011-209702 (PatentDocument 1) discloses a technique of reducing transmission loss relatingto a multi-core optical fiber (hereinafter simply referred to as “MCF”)in which a plurality of cores are comprised of pure silica glass.Japanese Patent No. 5545236 (Patent Document 2) discloses a technique ofreducing transmission loss by adding alkali metal to the core. Inaddition, Japanese Patent Application Laid-Open No. S60-176004 (PatentDocument 3) discloses a structure in which a plurality of single-corefibers can be separated by adopting glass to which alkali metal that iseasily melted by acid or the like is added, as glass covering theoutermost circumferential surface of the single-core fibers forming abunch fiber. Tetsuya Hayashi, et al., “Physical interpretation ofintercore crosstalk in multicore fiber: effects of macrobend, structurefluctuation, and microbend”, OPTICS EXPRESS Vol. 21, No. 5, pp.5401-5412 (Mar. 11, 2013) (Non-Patent Document 1) has descriptionrelating to transmission loss of an MCF, and Tetsuya Hayashi et al.,“Coupled-Core Multi-Core Fiber: High-Spatial-Density OpticalTransmission Fibers with Low Differential Modal Properties”, ECOC2015,We. 1. 4. 1. (2015) (Non-Patent Document 2) has description relating totransmission loss of an MCF manufactured by a rod-in collapse method.

Non-Patent Document 1 described above describes the definition of thepower coupling coefficient of adjacent cores. Non-Patent Document 2described above describes a coupled MCR In addition, R. Ryf, et al.,“1705-km Transmission over Coupled Core Fibre Supporting 6 SpatialModes”, ECOC2014, PD. 3. 2., Cannes-France (2014) (Non-Patent Document3) discloses a test result of mode division multiplex transmission towhich a coupled MCF is applied, and Beril Inan, et al., “DSP complexityof mode-division multiplexed receivers”, OPTICS EXPRESS Vol. 20, No. 9,pp. 10859-10869, (Apr. 23, 2012) (Non-Patent Document 4) discloses amulti-input-multi-output (hereinafter referred to as MIMO(Multi-Input-Multi-Output)) technique to enable mode multiplexing andmode division.

SUMMARY

The inventors have investigated conventional MCFs, and consequently havefound the following problem.

Specifically, it is well known that adding alkali metal elements to thecore is effective for reduction in loss. However, alkali metal elementsare easily diffused in comparison with other elements, and the alkalimetal concentration in the core in the optical fiber reduces incomparison with the concentration in a preform, during drawing from thepreform to the optical fiber. Accordingly, the alkali metalconcentration capable of contributing to structural relaxation of thecore glass in drawing is limited to some extent. In particular, in theconventional art described above in which alkali metal is added to onlyone core, alkali metal of high concentration cannot be added, to preventcrystallization of the glass region corresponding to the core. In an MCFin which a plurality of cores can be closely arranged, addition ofalkali metal is advantageous for reduction in loss, because alkali metalcan be easily diffused. However, in a non-coupled MCF, when a distance(hereinafter referred to as “core pitch”) between centers of adjacentcores is too small, crosstalk (hereinafter referred to as “XT”) occursbetween cores, and thus reduction in transmission loss is also limited.

The present invention has been made to solve the problem as describedabove. An object of the present invention is to provide a coupled MCF(CMCF) including a structure for achieving more efficient reduction intransmission loss by suppressing decrease in concentration caused bydiffusion of alkali metal. In a CMCF, the core pitch is intentionallyreduced, to generate XT (inter-core XT) between signals transmittedthrough the respective cores. The signals between which XT occurs aredecoded by MIMO processing disclosed in Non-patent Document 4 describedabove (see Non-patent Documents 2 and 3 described above). For thisreason, mode division multiplex transmission is enabled with atransmission system in which the CMCF is combined with MIMO processing.Because a CMCF actively generating inter-core XT as described above canbe designed with a narrower core pitch than that of a non-coupled MCF,alkali metal can easily be diffused mutually between adjacent cores, andthe CMCF is expected to suppress reduction in concentration caused bydiffusion of alkali metal (achievement of low transmission loss). Basedon the consideration described above, the inventors have found a corepitch and stress profiles thereof effective for reducing transmissionloss in the CMCF.

A CMCF according to the present embodiment comprises a plurality ofcores each extending along a predetermined direction, and a singlecladding covering the respective cores. In particular, each of the coresincludes alkali metal of a predetermined concentration contributing toreduction in transmission loss. In addition, to maintain an opticalcoupling state between adjacent cores among the cores, a power couplingcoefficient h between the adjacent cores is set to 1×10⁻³/m or more. Inaddition, to achieve marked reduction in transmission loss, a maximumvalue σ_ _(max) of stress profile on a line segment connecting centersof the adjacent cores has a negative value (compressive stress).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are diagrams illustrating cross-sectional structures,refractive index profiles, and alkali metal concentration profiles of aCMCF and a preform according to a first embodiment, respectively.

FIGS. 2A and 2B are diagrams for explaining relation between therefractive index profile and the stress profile of the CMCF according tothe first embodiment.

FIGS. 3A to 3G are diagrams illustrating various refractive indexprofiles applicable to a region R1 including a core and part of claddingaround the core.

FIGS. 4A to 4D are cross-sectional views of the CMCF, illustratingvarious examples of applicable core arrangement.

FIG. 5 is a table illustrating optical properties of three samples to 3(CMCF1 to CMCF3) of the CMCF according to the first embodiment, and asingle-core fiber (hereinafter referred to as SCF (single-core opticalfiber)) serving as a comparative example thereof.

FIGS. 6A and 6B are graphs illustrating relation between the maximumvalue σ_ _(max) (MPa) of the stress profile between adjacent cores andamount of decrease in transmission loss (dB/km), and relation betweenthe core pitch (distance between centers of adjacent cores) Λ_(core)(μm) and amount of decrease in transmission loss (dB/km), respectively,for each of the three samples 1 to 3 (MCF1 to MCF3) of the CMCFaccording to the first embodiment.

FIGS. 7A to 7C are diagrams illustrating cross-sectional structures,refractive index profiles, and alkali metal concentration profiles of aCMCF and a preform according to a second embodiment, respectively.

DETAILED DESCRIPTION

Explanation of Embodiments of Present Invention

First, details of embodiments of the present invention will beindividually explained hereinafter.

(1) A CMCF (coupled multicore optical fiber) according to the presentembodiment comprises a plurality of cores each extending along apredetermined direction, and a single cladding covering the respectivecores, as an aspect thereof In particular, each of the cores includesalkali metal of a predetermined concentration contributing to reductionin transmission loss. In addition, to maintain an optical coupling statebetween adjacent cores among the cores, a power coupling coefficient hbetween the adjacent cores is set to 1×10⁻³/m or more. In addition, toachieve marked reduction in transmission loss, a maximum value σ_ _(max)of a stress profile on a line segment connecting centers of the adjacentcores has a negative value. Specifically, compressive stress remains inthe cladding positioned between adjacent cores. The maximum value σ__(max) being positive means that tensile stress remains in the claddingpositioned between adjacent cores. Each of the cores includes one or atleast two of alkali metals selected from the group consisting oflithium, sodium, potassium, and rubidium.

The optical fiber according to the aspect described above is a CMCF, anda transmission medium applied to a transmission system decoding each ofsignals by MIMO processing, even when XT (inter-core XT) occurs betweenthe signals transmitted through the respective cores. Such a CMCF isrequired to maintain differential group delay (DGD) between signals tobe small, to reduce the load of MIMO processing. For this reason, in aCMCF, the core pitch is narrowed to actively generate XT betweensignals. A CMCF is designed to mix signals by actively generating XTbetween signals, to substantially reduce DGD between signals (seeNon-patent Document 3). The core pitch depends on the refractive indexstructure of each of the cores. To sufficiently generate XT betweensignals, the power coupling coefficient h between adjacent cores ispreferably set to 1×10⁻³/m (see Non-patent Document 2).

To efficiency reduce transmission loss, a plurality of alkali metaladded regions are provided in the preform of the CMCF before drawing,and alkali metal is required to be diffused between the alkali metaladded regions during drawing of the preform. In this state, viscosity ofglass around the cores reduces due to diffusion of alkali metal, and aregion in which compressive stress remains is provided in the CMCF afterdrawing. For example, in the preform of the CMCF, with the structure inwhich each of a plurality of core portions includes an alkali metaladded region, a cladding disposed between adjacent cores serves as analkali metal diffusion region in the CMCF after drawing. Forming aplurality of alkali metal added regions in the preform of the CMCFbefore drawing as described above prevents decrease in alkali metalconcentration during drawing, and produces a CMCF with an efficientlyreduced transmission loss.

As an aspect of the present embodiment, the maximum value σ_ _(max) ofthe stress profile is preferably −20 MPa or less.

(3) As an aspect of the present embodiment, the core pitch Λ_(core) ispreferably 35 μm or less.

(4) As an aspect of the present embodiment, in the preform of the CMCFbefore drawing, a region corresponding to the cladding may be providedwith at least one alkali metal added region. In this case, in the CMCFafter drawing, a distance Λ_(core-clad) between the diffusion centerposition corresponding to the central position of the alkali metal addedregion in the preform and the central position of the core adjacent tothe diffusion center position among the cores is preferably 45 μm orless. More preferably, the distance is 30 μm or less, and furtherpreferably, the distance is 25 μm or less.

(5) As an aspect of the present embodiment, each of the cores iscomprised of SiO₂ glass in which a concentration of GeO₂ molecules isset to be 0 wt % or more to 1 wt % or less. Namely, each of the coresdoes not include GeO₂ molecules or includes GeO₂ molecules having aconcentration of 1 wt % or less. In the structure in which fluorine isadded to the cladding, transmission loss of each of the cores ispreferably 0.16 dB/km or less at a wavelength of 1550 nm. Morepreferably, the transmission loss at the wavelength of 1550 nm is 0.155dB/km or less, and further preferably 0.150 dB/km or less.

(6) As an aspect of the present embodiment, in the structure in whichgermanium of predetermined concentration is added to at least one of thecores, the transmission of the core doped with the germanium ispreferably 0.18 dB/km or less at the wavelength of 1550 nm.

(7) As an aspect of the present embodiment, average concentration ofalkali metal in each of the cores is preferably 0.2 atom ppm or more and50 atom ppm or less.

(8) As an aspect of the present embodiment, in the preform of the CMCFbefore drawing, average concentration of halogen elements in each ofregions corresponding to the cores is preferably 1000 atom ppm or moreand 30000 atom ppm or less.

(9) As an aspect of the present embodiment, concentration of alkalimetal in the surface of the cladding is preferably 1 atom ppm or less.

Each of the aspects mentioned in the column of “Explanation ofEmbodiments of Present Invention” is applicable to each of the otheraspects, or all the combinations of the other aspects.

Details of Embodiments of Present Invention

Specific examples of a CMCF according to the present invention will bedescribed in detail hereinafter with reference to attached drawings. Thepresent invention is not limited to these examples, but is intended toinclude all changes within the meanings and ranges equivalent to theclaims. In the explanation of the drawings, the same elements aredenoted by the same reference numerals, and overlapping explanation isomitted.

First Embodiment

FIG. 1A illustrates a cross-sectional structure of a preform 100A beforedrawing for manufacturing a CMCF 200A according to a first embodiment.The cross section illustrated in FIG. 1A is a cross section orthogonalto a central axis AX (agreeing with the longitudinal direction of thepreform 100A) of the preform 100A. The preform 100A includes coreportions 110 each extending along the central axis AX from one end A tothe other end B, and a cladding portion 120 covering each of the coreportions 110. In the cross section of FIG. 1A, three core portions 110are arranged to surround the central axis AX, as an example. The CMCF200A with the cross section illustrated in FIG. 1A is obtained bydrawing the preform 100A, and its cross-sectional structure is similarto the cross-sectional structure of the preform 100A. Specifically,cores 210 of the CMCF 200A obtained by drawing the preform 100Acorrespond to the core portions 110 of the preform 100A, and a cladding220 of the CMCF 200A corresponds to the cladding portion 120 of thepreform 100A. As an example of arrangement of cores, FIG. 1A illustratesa structure in which three core portions 110 (or cores 210) are arrangedaround the central axis AX of the preform 100A (or the CMCF 200A), butvarious core arrangements may be applied to the present embodiment asdescribed later, and the core arrangement is not limited to the exampleof FIG. 1A.

FIG. 1B illustrates a refractive index profile 150A and an alkali metalconcentration profile 160A of the preform 100A along a line L (linerunning through the centers of the adjacent core portions 110) in FIG.1A. As can be seen from FIG. 1B, in the present embodiment, each of thecore portions 110 of the preform 100A serves as an alkali metal addedregion to which alkali metal is added.

FIG. 1C illustrates a refractive index profile 250A and an alkali metalconcentration profile 260A of a CMCF 200A obtained by drawing thepreform 100A according to the present embodiment. The profiles areprofiles along line L in FIG. 1A, like FIG. 1B.

In addition, FIGS. 2A and 2B are diagrams for explaining relationbetween the refractive index profile 250A of the CMCF 200A illustratedin FIG. 1C and a stress profile 255. In particular, FIG. 2A illustratesthe refractive index profile 250A of the CMCF 200A illustrated in FIG.1C, and FIG. 2B illustrates the stress profile 255 associated with therefractive index profile 250A at positions on the line L in FIG. 1A.When each of the cores 210 of the preform 100A serve as alkali metaladded region as illustrated in FIG. 1B, alkali metal are diffused fromeach of the cores 210 in the CMCF 200A obtained after drawing, andalkali metal exists also between the adjacent cores 210. In this state,the viscosity of glass around each core 210 decreases, and the stressprofile 255 with the shape as illustrated in FIG. 2B is obtained. In thespecification of the present application, σ_ _(max) indicates themaximum value of the stress profile on the line segment connecting thecenters of the adjacent core portions 110 in the CMCF 200A.Specifically, because the maximum value σ_ _(max) of the stress profile255 illustrated in FIG. 2B has a negative value, a compressive stressremaining region is provided in the core 210 and the cladding 220disposed around the core 210.

Generally, in the preform 100A provided with core portions 110 to eachof which alkali metal is added, alkali metals in each of the coreportions 110 are mutually diffused, because the preform 100A is heatedduring drawing. This structure decreases the alkali metal concentrationin each of the cores 210 in the CMCF 200A after drawing, as illustratedin FIG. 1C. However, when the core pitch Λ_(core) reduces, alkali metaldiffused from a core flows into the adjacent core, to ease decrease inconcentration of alkali metal, and promote relaxation of the glassstructure. With this structure, the CMCF 200A according to the presentembodiment enables reduction in transmission loss. In addition, the CMCF200A is expected to reduce transmission loss in comparison with the caseof an SCF (see Non-patent Document 2). To efficiently diffuse alkalimetal mutually between the cores 210, the adjacent cores are preferablyclose to each other (the core pitch Λ_(core) is small). By contrast, ina non-coupled MCF, inter-core XT increases. When the inter-core XT islarge, increase in transmission loss also occurs due to the inter-coreXT in a non-coupled MCF, and the effect of addition of alkali metal islimited.

By contrast, the CMCF 200A according to the present embodiment isdesigned to substantially reduce DGD between signals to reduce the loadof the MIMO processing. To sufficiently generate XT between signals, thepower coupling coefficient h between adjacent cores is designed to be1×10⁻³/m or more.

As disclosed in Non-patent Document 1 described above, for example, theoptical power Pn moving from a core in to a core n per unit length canbe expressed with following Expression (1).dP _(n) /dz=h(P _(m) −P _(n))   (1)where h is the power coupling coefficient, and means efficiency withwhich the optical powers are coupled in the adjacent cores (defining thecoupled core) with respect to the CMCF. The power coupling coefficient hin Expression (1) described above means a power coupling coefficienth_(mn) from the core m to the core n, and is calculated from therefractive index of each of the cores, the core pitch, and the bendingstate of the CMCF. Specifically, the power coupling coefficient h(=h_(mn)) is defined by following Expression (2), in the same manner asExpression (8) of Non-patent Document 1 described above, and the wholedescription of Non-patent Document 1 is incorporated into thespecification of the present application. The power coupling coefficienth_(nm) disclosed in Expression (8) of Non-patent Document 1 is a powercoupling coefficient from the core n to the core m, but is equivalent tothe power coupling coefficient h of Expression (1) described aboveserving as the power coupling coefficient h_(mn), from the core m to thecore n.h(=h _(mn))=ΔX _(mn) /Δz   (2)where Δz is a segment length of a fiber segment [z₁, z₂] (fiber segmentbetween a position of a distance z₁ along a fiber longitudinal directionfrom a reference point such as a light incident end surface and aposition of a distance z₂ (>z₁) from the reference point), and isprovided by “z₂−z₁”. ΔX_(mn) is an average crosstalk increase from thecore m to the core n in the fiber region with the segment length Δz, andprovided by Expression (4) of Non-patent Document 1 described above.

The alkali metal addition concentration of the core portion 110 in thepreform 100A before drawing is set such that the average concentrationof alkali metal in each core 210 in the CMCF 200A after drawing is 0.2atom ppm or more and 50 atom ppm or less. This is becausecrystallization of SiO₂ glass to which alkali metal of highconcentration is added is accelerated, and relaxation of the glassstructure of each core 210 can be promoted during drawing, when theaverage value of alkali metal included in each core 210 of the CMCF 200Aafter drawing is 0.2 atom ppm or more. By contrast, when the averageconcentration of alkali metal in each core 210 in the CMCF 200A afterdrawing becomes 50 atom ppm or more, the transmission loss at thewavelength of 1550 nm after irradiation of radial rays with cumulativeabsorption dose of 0.10 Gy or more increases by 0.02 dB/km or more incomparison with that before irradiation of radial rays, and a problemoccurs in use in a system requiring long-time stability of transmissionloss, such as a submarine system. For this reason, such averageconcentration is not preferable.

In addition, when the core portions 110 of the preform 100A has lowhalogen element concentration, such low halogen element concentrationfails to sufficiently obtain the impurity removal effect with halogenelements in the preform manufacturing process, and causes increase intransmission loss due to absorption of impurities. By contrast, too manyhalogen elements are not desirable, because a halogen compound of alkalimetal is generated and functions as a crystal generation core. Bysetting the concentration of halogen elements, such as Cl and F, to arange of 1000 atom ppm or more and 30000 atom ppm or less, the problemdescribed above is not caused, and a CMCF of low transmission loss canbe obtained.

In addition, the concentration of alkali metal in the glass surface(surface of the cladding 220) in the CMCF 200A after drawing is 1 atomppm or less. When alkali metal is diffused to the glass surface of theCMCF 200A after drawing, the effect of cutting the SiO₂ glass network bythe alkali metal causes large decrease in mechanical fatiguecoefficient, and causes a problem in practical use. To avoid thisproblem, the concentration of alkali metal to the glass surface of theCMCF 200A after drawing is preferably 1 atom ppm or less, morepreferably 0.1 atom ppm or less. When potassium is added as alkali metalto the core portions 110 of the preform 100A, potassium is diffused in arange with a radius of 15 to 50 μm after drawing. The diffusion radiusdepends on the potassium concentration at the stage of preform, and thetemperature history during drawing. Accordingly, it is desirable tocheek the potassium reach distance after drawing in advance, anddetermine the potassium addition position to the core portions 110 orthe cladding portions 120 in the cross section of the preform 100A.

Specifically, the average concentration of alkali metal elements (suchas potassium) added to the core portions 110 of the preform 100A toreduce the transmission loss is 5 atom ppm or more, preferably 50 atomppm or less. Because increase in loss due to irradiation of radial raysincreases as the potassium concentration increases, the upper limit ofthe potassium average concentration in the core portions 110 at thestage of the preform is preferably set to 500 atom ppm. In addition, inthe drawing process from the preform 100A to the CMCF 200A, the time forwhich each position of the preform 100A is maintained at 1500° C. ormore in a drawing furnace is 110 minutes or less, and the drawingvelocity (drawing velocity) is preferably 1200 m/min or more, morepreferably 1500 to 2300 m/min or more. The diameter of the preform 100Ais preferably 70 to 170 mmϕ, more preferably 90 to 150 mmϕ.

The transmission loss of the CMCF 200A after drawing at the wavelengthof 1.55 μm decreases, as the time for which each position of the preform100A is maintained at a temperature of 1500° C. or more decreases. Thisphenomenon is considered to be caused by the following reason.Specifically, when potassium with an average concentration of 500 atomppm or less is included in the core portions 110 of the preform 100A, ahypothetical temperature of the CMCF 200A obtained from the preform 100Ais 1400 to 1550° C., and diffusion of potassium advances in the timewith the temperature from the peak temperature (more than 1500° C.) to1500° C. in the drawing furnace. By contrast, when diffusion ofpotassium advances too much, potassium is broadly diffused outsidebeyond the optical power profile f the communication wavelength band(1550 nm band). In this case, because the effective potassiumconcentration decreases, structural relaxation of the glass network doesnot advance, and the transmission loss is not reduced. Accordingly, theoptical transmission loss of the CMCF 200A after drawing is morereduced, as the time for which the preform is maintained at the glasstemperature of 1500° C. or more, in which diffusion of potassiumadvances well, is shorter.

By drawing from the preform 100A to the CMCF 200A under such conditions,the alkali metal concentration of 0.2 atom ppm or more can be preferablyachieved, in each of the cores 210 of the CMCF 200A after drawing. In aCMCF in which each core is substantially comprised of pure silica glass,to achieve the transmission loss to be lower than 0.170 dB/km, the timefor which each position of the preform is maintained at a temperature of1500° C. or more is required to be 110 minutes or less, more preferably70 minutes or less.

In addition, the present embodiment enables, by a preferablecombination, slow-cooling drawing promoting relaxation of the glassstructure by maintaining the temperature of the CMCF during drawing at acertain temperature or more. When the present embodiment is combinedwith slow-cooling drawing, further lower transmission loss can beobtained. For a slow-cooling drawing method, the skilled person couldobtain proper manufacturing conditions necessary for reducingtransmission loss.

In addition, in the specification of the present application, “atom ppm”indicating the alkali concentration and the halogen concentrationindicates number of dopant atoms in SiO₂ glass of million units. Forexample, in the case of potassium, the term “atom ppm” indicates a ratioof the number of atoms of K to the number of SiO₂ molecules, regardlessof the coupling form in the SiO₂ glass. The same is applicable to thecase of using Li, Na, or Rb, and the case of using Cl or F.

Various refractive index profiles as illustrated in FIGS. 3A to 3G areapplicable to the region R1 (see FIG. 1A) including each core 210 andpart of the cladding 220 around the core 210 in the CMCF 200A having thestructure as described above.

A proper structure may be selected according to use, with respect to therefractive index profile of each core 210 and optical propertiesaccompanying therewith. The cores 210 may have a uniform structure, ordifferent structures. In addition, the number of cores in the crosssection of the CMCF 200A is not limited, and the cross-sectionaldiameter (glass diameter) of the CMCF 200A and the external diameter ofthe covering resin provided on the external circumferential surface ofthe cladding 220 may be properly set according to the number of corescontained.

Specifically, as the shape of the refractive index profile of the regionR1 including each core 210, any of a step type (FIG. 3A), a ring type(FIG. 3B), a double-step type (FIG. 3C), and a graded type (FIG. 3D) isapplied to the region corresponding to the core 210. In addition, any ofa depressed type (FIG. 3E), a matched type (FIG. 3F), and a trench type(FIG. 3G) is applicable to the region corresponding to the cladding 220.Each core 210 may have a structure premised on the single mode operationin which the number of modes to be transmitted through the core is one,or a structure premised on the multi-mode operation transmitting aplurality of modes.

FIGS. 4A to 4D are cross-sectional views of CMCFs illustrating variousexamples of core arrangements applicable to the present embodiment.Specifically, as the core arrangement applicable to the presentembodiment, structures in which cores 210 are arranged symmetricallywith respect to the central axis AX of the CMCF 200A may be adopted, asillustrated in FIGS. 4A to 4C, As another example, a structure asillustrated in FIG. 4D may be adopted. In the structure of FIG. 4D, aplurality of core groups, each of which is constituted by a plurality ofcore elements, are arranged in a ring shape around the central axis AXof the CMCF 200A.

In the refractive index profile as described above, when a structure isadopted in which at least one of the cores 210 is comprised of SiO₂,glass with GeO₂ molecules having an additive amount of 1 wt % or lessand fluorine is doped to the cladding 220, the transmission loss of thecore 210 is preferably 0.16 db/km or less at the wavelength of 1550 mn.Generally, when each core is comprised of SiO₂ glass to which GeO₂molecules to increase the refractive index of the core are notsubstantially doped, because dispersion caused by concentrationfluctuations of GeO₂ molecules can be suppressed, the transmission losscan be suppressed to 0.16 dB/kin or less. The transmission loss ispreferably 0.155 dB/km or less, more preferably 0.150 dB/km or less.

In addition, in the refractive index profile as described above, astructure may be adopted in which germanium is doped to at least one ofthe cores 210. In this case, the transmission loss of each of the coresis preferably 0.18 dB/km at the wavelength of 1550 nm. A core to whichgermanium is doped generally has higher transmission loss than that ofan optical fiber including a pure silica core. However, dispersion isreduced by co-doping the core with germanium and alkali metal, andconsequently transmission loss is reduced. However, when alkali metal isdoped to a core portion to which germanium is doped at the stage of thepreform, crystals are easily generated in the core after drawing, andmanufacturability deteriorates. For this reason, in the preformmanufacturing process, it is preferable that no alkali metal is directlydoped to the core portion doped with germanium, but alkali metal isdoped to only the cladding portion of the preform, and the alkali metalis diffused into the core in heating in drawing.

The following is detailed explanation of optical properties and the likeof samples of the CMCF 200A according to the present embodiment and acomparative example, with reference to FIGS. 5, 6A, and 6B.

As prepared samples of the CMCF 200A according to the presentembodiment, each of CMCF1 to CMCF3 has a cross-sectional structureincluding three cores as illustrated in FIG. 1A, and the periphery ofeach core has the refractive index profile (step type) as illustrated inFIG. 3A. The core arrangement adopted in each of CMCF1 to CMCF3 has astructure in which three cores 210 are arranged around the central axisAX, as illustrated in FIG. 1A. By contrast, the comparative example isan SCF including a core, and has the refractive index profile asillustrated in FIG. 3A.

Each of CMCF1 serving as Sample 1, CMCF2 serving as Sample 2, CMCF3serving as Sample 3, and the SCF of the comparative example is based onpure silica, and has a 0.32% relative refractive index difference Δ ofthe core center based on the cladding, and has a core diameter 2a of11.1 μm. In addition, CMCF1 has the core pitch Λ_(core) of 28 μm, andthe power coupling coefficient h of 1.1×10⁻²/m. CMCF2 has the core pitchΛ_(core) of 32 μm, and the power coupling coefficient h of 1.0×10⁻³/m.CMCF3 has the core pitch Λ_(core) of 38 μm and the power couplingcoefficient h of 8.5×10⁻⁶/m. Each of CMCF1, CMCF2, CMCF3, and the SCF ofthe comparative example has a fiber external diameter of 125 μm, and hasdrawing conditions in which the drawing velocity is 1300 m/min, and thedrawing tension is 80 to 100 g. Potassium is doped as alkali metaldopant to each core portion of the respective preforms of CMCF1, CMCF2,CMCF3, and the SCF.

FIG. 5 is a table illustrating optical properties of CMCF1, CMCF2,CMCF3, and the SCF together.

Specifically, the average effective area A_(eff) of the SCF prepared asdescribed above was 110 μm² at the wavelength of 1550 nm. In addition,the average effective area A_(eff) of each core of the CMCF1(Λ_(core)=28 μm) was 107 μm² at the wavelength of 1550 nm, and thestress maximum value σ_ _(max) between cores of CMCF1 was −28 MPa(compressive stress). The average effective area A_(eff) of each core ofthe CMCF2 (Λ_(core)=32 μm) was 109 μm² at the wavelength of 1550 nm, andthe stress maximum value σ_ _(max) between cores of CMCF2 was −20 MPa(compressive stress). The average effective area A_(eff) of each core ofthe CMCF3 (Λ_(core)=38 μm) was 105 μm² at the wavelength of 1550 nm, andthe stress maximum value σ_ _(max) between cores of CMCF3 was MPa(tensile stress).

The transmission loss of the SCF of the comparative example was 0.161 dBat the wavelength of 1550 nm. Under the same drawing conditions,transmission loss of the CMCF1 was 0.148 dB at the wavelength of 1550nm, transmission loss of the CMCF2 was 0.153 dB at the wavelength of1550 nm, and transmission loss of the CMCF3 was 0.158 dB at thewavelength of 1550 nm. In comparison with the transmission loss of theSCF, an amount of decrease in transmission loss of CMCF1 was −0.013dB/km, an amount of decrease in transmission loss of CMCF2 was −0.008dB/km, and an amount of decrease in transmission loss of CMCF3 was−0.003 dB/km.

FIG. 6A is a graph illustrating relation between the maximum value σ__(max) (MPa) of the stress profile between adjacent cores and the amountof decrease (dB/km) in transmission loss of each of various CMCF samplesincluding Samples 1 to 3 (CMCF1 to CMCF3) prepared as described above,

As can be seen from FIG. 6A, a marked decrease in transmission loss canbe found in samples (including CMCF1 and CMCF2) in which the stressmaximum value σ_ _(max) has a negative value, that is, compressivestress is maintained. More preferably, the stress maximum value σ__(max) is set to −20 MPa or less, to further reduce the transmissionloss. Further preferably, the stress maximum value σ_ _(max) is set to−30 MPa or less.

FIG. 6B is a graph illustrating relation between the core pitch Λ_(core)(μm) and the amount of decrease in transmission loss (dB/km) of each ofvarious CMCF samples including Samples 1 to 3 (CMCF1 to CMCF3) preparedas described above.

As can be seen from FIG. 6B, in a sample with a core pitch Λ_(core)exceeding 35 μm like CMCF3, the amount of decrease in transmission lossis less than 0.005 dB/km. By contrast, in a sample with a core pitchΛ_(core) equal to or less than 35 μm like CMCF1 and CMCF2, the amount ofdecrease in transmission loss is 0.005 dB/km or more. The core pitchΛ_(core) is preferably 30 μm or less, more preferably 25 μm or less.

Second Embodiment

FIGS. 7A to 7C are diagrams illustrating cross-sectional structures,refractive index profiles, and alkali metal concentration profiles of aCMCF 200B and a preform 100B according to a second embodiment,respectively. The second embodiment illustrated in FIGS. 7A to 7C hasthe same structure as the structure FIG. 1A) of the first embodiment,except that an alkali-metal-doped region 500 is provided also in acladding portion 120 held between core portions 110, as well as two coreportions 110, in the preform 100B for manufacturing the CMCF 200B. Inthe example of FIG. 7A, a middle position between the cores 110 adjacenton line L is set to a diffusion center position O in which theconcentration of alkali metal has a local maximum value.

Specifically, in FIG. 7A, the preform 100B includes core portions 110each extending along the central axis AX from one end A to the other endB, and a cladding portion 120 covering each of the core portions 110. Asan example, three core portions 110 are arranged to surround the centralaxis AX, on the cross section of FIG. 7A. The CMCF 200B according to thepresent embodiment is obtained by drawing the preform 100B, and has across-sectional structure similar to the cross-sectional structure ofthe preform 100B. Cores 210 of the CMCF 200B correspond to the coreportions 110 of the preform 100B, and a cladding 220 of the CMCF 200Bcorresponds to the cladding portion 120 of the preform 100B. FIG. 7B isa diagram illustrating a refractive index profile 150B and an alkalimetal concentration profile 160B of the preform 100B along line L inFIG. 7A. As can be seen from FIG. 7B, in the present embodiment, analkali-metal-doped region 500 doped with alkali metal is provided alsoin the cladding portion 120 of the preform 100B. FIG. 7B does notillustrate alkali metal concentration profile in one of the coreportions 110, but an alkali-metal-doped region may be provided in allthe core portions 110 also in the present embodiment, as a matter ofcourse. FIG. 7C illustrates a refractive index profile 250B and analkali metal concentration profile 260B of the CMCF 200B obtained bydrawing the preform 100B according to the present embodiment, andillustrates profiles along line L in FIG. 7A, in the same manner as FIG.7B. The term “Λ_(core-clad)” illustrated in FIG. 7C indicates a distancebetween the diffusion center position O in which the concentration ofthe alkali metal in the CMCF 200B after drawing has a local maximumvalue and the central position of each core 210.

The present embodiment enables more efficient reduction in transmissionloss, by adding alkali metal also to the cladding portion 120, as wellas the core portions 110 of the preform 100B. However, when a distancebetween the alkali-metal-doped region and the core portion 110 is large,alkali metal is not diffused to the core during drawing, and notransmission loss reduction effect can be obtained. Accordingly, in thecase of using substance with an atomic number less than potassium (K),as the alkali metal, the distance Λ_(core-clad) between the center ofthe core 210 and the diffusion center position O corresponding to thecenter of the alkali-metal-doped region 500 is required to be 45 μm orless, in the CMCF 200B after drawing. More preferably, the distanceΛ_(core-clad) is 30 μm or less, more preferably 25 μm or less.

In addition, when alkali metal is doped to the core portions 110 of thepreform 100B, crystals easily occur in the cores during drawing, andincrease in transmission loss may occur due to mixing of impuritiesother than alkali metal into the core portions 110, in the step ofadding alkali metal into the core portions 110. For this reason, alkalimetal is not directly doped to the core portions 110 at the stage ofpreform, but alkali metal (alkali-metal-doped region 500) doped to thecladding portion 120 in the drawing process is diffused into the coreduring drawing. This structure enables low transmission loss, withoutcrystallization, fear of excessive loss due to mixing of impurities, orreduction in production yield.

In the preform 100B, with respect to arrangement of thealkali-meal-doped regions in the core portions 110 and the claddingportion 120, alkali-metal-doped regions exist in both core portions 110and a middle region therebetween and extend along the preformlongitudinal direction (central axis AX). This structure enablesreduction in transmission loss of the obtained CMCF, even when the corepitch Λ_(core) is set larger than that of the first embodiment in whicheach of adjacent core portions 110 is set as alkali-metal-doped region.However, when alkali metal is provided in a region close to the externalcircumference of the cladding, such arrangement is not desirable becauseit causes increase in concentration of alkali metal at the externalcircumference of the fiber due to diffusion during drawing, and reducesmechanical strength of the obtained CMCF 200B. Accordingly, theconcentration of alkali metal in the surface of the cladding 220 of theCMCF 200B is preferably 1 atom ppm or less.

As described above, the present embodiment enables a coupled MCF (CMCF)achieving more efficient reduction in transmission loss, by suppressingdecrease in concentration of alkali metal.

What is claimed is:
 1. A coupled multi-core optical fiber comprising: aplurality of cores extending from one end to the other end; and a singlecladding covering each of the cores, wherein each of the plurality ofcores includes alkali metal contributing to reduction in transmissionloss, and stress on a line segment connecting centers of the adjacentcores is compressive stress.
 2. The coupled multi-core optical fiberaccording to claim 1, wherein a maximum value σ_ _(max) of the stress onthe line segment is −20 MPa or less.
 3. The coupled multi-core opticalfiber according to claim 1, wherein a distance Λ_(core) between theadjacent cores is 35 μm or less.
 4. A coupled multi-core optical fibercomprising: a plurality of cores extending from one end to the otherend; and a single cladding covering each of the cores, wherein each ofthe plurality of cores includes alkali metal contributing to reductionin transmission loss, stress on a line segment connecting centers of theadjacent cores is compressive stress, and the cladding includes adiffusion center position in which concentration of the alkali metal hasa local maximum value, and a distance Λ_(core-clad) between thediffusion center position and a center position of a core adjacent tothe diffusion center position among the plurality of cores is 45 μm orless.
 5. The coupled multi-core optical fiber according to claim 1,wherein each of the plurality of cores is comprised of SiO₂ glass inwhich a concentration of GeO₂ molecules is set to be 0 wt % or more to 1wt % or less, fluorine is doped to the cladding, and transmission lossof each of the plurality of cores at a wavelength of 1550 nm is 0.16dB/km or less.
 6. The coupled multi-core optical fiber according toclaim 1, wherein germanium is doped to at least one of the plurality ofcores, and transmission loss of the core with added germanium is 0.18dB/km or less at the wavelength of 1550 nm.
 7. The coupled multi-coreoptical fiber according to claim 1, wherein average concentration of thealkali metal in each of the plurality of cores is 0.2 atom ppm or moreand 50 atom ppm or less.
 8. The coupled multi-core optical fiberaccording to claim 1, wherein average concentration of halogen elementsin each of regions corresponding to the plurality of cores in a preformof the coupled multi-core optical fiber before drawing is 1000 atom ppmor more and 30000 atom ppm or less.
 9. The coupled multi-core opticalfiber according to claim 1, wherein concentration of the alkali metal ina surface of the cladding is 1 atom ppm or less.
 10. The coupledmulti-core optical fiber according to claims 4, wherein a maximum valueσ_ _(max) of the stress on the line segment is −20 MPa or less.
 11. Thecoupled multi-core optical fiber according to claims 4, wherein adistance Λ_(core) between the adjacent is 35 μm or less.
 12. The coupledmulti-core optical fiber according to claims 4, wherein each of theplurality of cores is comprised of SIO₂ glass in which a concentrationof Geo₂ molecules is set to be 0 wt % or more to 1 wt % or less,fluorine is doped to the cladding, and transmission loss of each of theplurality of cores at a wavelength of 1550 nm in 0.16 dB/km or less. 13.The coupled multi-core optical fiber according to claims 4, whereingermanium is doped to at least one of the plurality of cores, andtransmission loss of the core with added germanium is 0.18 dB/km or lessat the wavelength of 1550 nm.
 14. The coupled multi-core optical fiberaccording to claims 4, wherein average concentration of the alkali metalin each of the plurality of cores is 0.2 atom ppm or more and 50 atomppm or less.
 15. The coupled multi-core optical fiber according toclaims 4, wherein average concentration of halogen elements in each ofregions corresponding to the plurality of cores in a preform of thecoupled multi-core optical fiber before drawing is 1000 atom ppm or moreand 30000 atom ppm of less.
 16. The coupled multi-core optical fiberaccording to claims 4, wherein concentration of the alkali metal in asurface of the cladding is 1 atom ppm or less.